Palladium-catalyzed hydrophosphonylation of alkenes with dialkyl H-phosphonates

Article abstract of DOI:10.1002/adsc.201400190

The palladium-catalyzed hydrophosphonylation of alkenes with previously unreactive acyclic dialkyl H-phosphonates has been developed. A catalyst system based on palladium/DavePhos or palladium/SPhos enables the transformation of various alkenes to phospho

Full text of DOI:10.1002/adsc.201400190

UPDATES
DOI: 10.1002/adsc.201400190
Palladium-Catalyzed Hydrophosphonylation of Alkenes with
Dialkyl H-Phosphonates
Mathieu Candy,a,* Sophie A. L. Rousseaux,a,* Alberto Cirugeda San Romꢀn,a
Monika Szymczyk,a,b Pawel Kafarski,b Eric Leclerc,a Emmanuel Vrancken,a
and Jean-Marc Campagnea
a
Institut Charles Gerhardt, UMR 5253 CNRS-UM2-UM1-ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5,
France
Fax : (+33)-(0)4-6714-4322; phone: (+33)-(0)4-6714-7221; e-mail: mathieu.candy26@orange.fr or
sophie.rousseaux@sjc.ox.ac.uk
b
´
Department of Bioorganic Chemistry, Faculty of Chemistry, Wrocław, University of Technology, Wybrzez˙e Wyspianskiego
27, 50-370 Wrocław, Poland
Received: February 20, 2014; Revised: April 6, 2014; Published online: June 20, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400190.
Abstract: The palladium-catalyzed hydrophospho-
nylation of alkenes with previously unreactive acy-
clic dialkyl H-phosphonates has been developed. A
catalyst system based on palladium/DavePhos or
palladium/SPhos enables the transformation of vari-
ous alkenes to phosphonates in moderate to good
yields and with excellent levels of regioselectivity.
Hydrophosphonylations with internal, disubstituted
and/or unactivated alkenes are also reported.
very attractive since these transformations are intrins-
ically atom-economical.[6] Moreover, this strategy may
provide tools for regio- and enantiocontrol in alkene
hydrophosphonylation, which is not possible using the
radical-mediated approach.
À
Over the last 20 years, the addition of H P(O)
bonds to alkynes has been well investigated and nu-
merous metal-catalyzed protocols have been estab-
lished for the highly selective synthesis of alkenyl-
phosphorus compounds.[6] In contrast, the addition of
À
H P(O) bonds to less reactive alkenes remains more
Keywords: acyclic dialkyl H-phosphonates; alkenes;
hydrophosphonylation; palladium catalysis; regiose-
lectivity
problematic. In this area, Montchamp and co-workers
have developed the hydrophosphinylation of alkenes
using alkyl phosphinate derivatives or aqueous phos-
phinic acid.[6,7] These authors found that catalyst sys-
tems based on Pd/Xantphos generally proved most ef-
ficient, giving access to H-phosphinate derivatives. It
Organophosphorus compounds are highly useful should be noted that these can be converted to phos-
building blocks in synthetic organic chemistry, for ex- phonates upon oxidation.[7a] On the other hand, hy-
ample, playing key roles in the stereoselective synthe- drophosphonylation processes have, thus far, re-
sis of olefins.[1,2] They have also found numerous ap- mained challenging and limited in scope. Pioneering
plications in the crop science and pharmaceutical in- work by the group of Tanaka has enabled the addition
dustries as well as being used as flame retardants, ex- of pinacol-derived H-phosphonate 1a to alkenes
tractants and ligands.[3] As a result of their impor- under palladium catalysis (Scheme 1).[8]
tance, wide interest has been generated in the
Terminal aliphatic alkenes exclusively yield the
development of new strategies for their preparation anti-Markovnikov b-adduct and styrene derivatives
which are complementary to the classical Michaelis– provide the Markovnikov a-adduct with excellent se-
Arbuzov and Michaelis–Becker reactions.[4] Radical- lectivity (95:5). Unfortunately, acyclic internal alkenes
based processes have proven to be both practical and are inert to these reaction conditions and less-strained
atom-economical.[4a–d] In recent years, transition cyclic alkenes react much less efficiently. Since this
metal-catalyzed reactions have emerged as highly effi- early report, examples of Pd- and Rh-catalyzed hy-
cient and selective tools for the preparation of drophosphonylations of dienes, allenes and cyclopro-
carbon-phosphorus bonds.[5] In particular, the devel- penes have been reported, as well as asymmetric var-
opment of metal-catalyzed methods for the addition iants of this transformation for norbornene and sty-
of H P(O) bonds to alkynes and alkenes has become rene derivatives.[5,9,10] However, in all cases, the reac-
À
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Mathieu Candy et al.
Scheme 1. Palladium-catalyzed hydrophosphonylation of alkenes.
tions are limited to the use of highly reactive pinacol-
derived H-phosphonate 1a and acyclic H-phosphonate
derivatives have, thus far, remained largely unreac-
tive.[11] As a result, the scope and synthetic utility of
alkene hydrophosphonylation reactions remain se-
verely limited. Moreover, the subsequent derivatiza-
tion of pinacol phosphonates can be problematic.
Indeed, the use of these compounds in Horner– phonates as substrates for the hydrophosphonylation
Emmons reactions is not possible and their hydrolysis of alkenes. The hydrophosphonylation of styrene (4a)
to more compatible acyclic phosphonates is diffi- with dibutyl H-phosphonate (1c) was chosen as
cult.[12] Finally, the pinacol derivative 1a is significant- a benchmark transformation to validate this hypothe-
ly more expensive than other acyclic H-phosphonates. sis (Table 1).[13] We initially examined a catalyst
New methods are therefore still highly desirable to system based on Pd/Xantphos combinations due to its
provide a more general and direct access to phospho- previous success in hydrophosphinylation reactions.[7]
nates via metal-catalyzed hydrophosphonylation. Unfortunately, under these conditions, low conversion
Herein, we describe the development of a palladium/ of 1c was observed and the hydrophosphonylation
dialkylbiarylphosphine catalyst system for the hydro- product could not be detected by 31P NMR of the
phosphonylation of alkenes with various, typically un- crude reaction mixture (Table 1, entry 1). We next ex-
reactive, H-phosphonates (Scheme 1).
amined the use of dialkylbiarylphosphines since these
Mechanistic investigations by Tanaka and co-work- ligands have been shown to enable numerous difficult
ers suggest that the transformation proceeds through: cross-coupling reactions and to promote reductive
(i) oxidative addition of Pd(0) into the P H bond, (ii) elimination processes.[14] It should be noted that
À
reversible hydropalladation of the alkene, and (iii) re- throughout the reaction optimization process, we
ductive elimination.[8] The same group has also dem- found that a water-mediated catalyst activation proto-
onstrated that the exceptional reactivity of pinacol- col was necessary to ensure reproducible yields of the
derived H-phosphonate 1a compared to other desired product.[15] A catalyst based on JohnPhos led
H-phosphonate derivatives is related to its ability to to 68% conversion of 1c and a 3:97 ratio of re-
accelerate the rate-determining reductive elimination gioisomers (entry 2). Contrary to previous palladium-
step. Indeed, the authors found that heating complex catalyzed alkene hydrophosphonylation protocols that
2a at 708C in benzene-d6 for one hour afforded prod- employ pinacol-derived H-phosphonate 1a,[8] a com-
uct 3a in 92% yield, while the analogous reaction plete change in selectivity for the anti-Markovnikov
with compound 2b was completely ineffective [Eq. b-adduct was observed in this case. Ligands contain-
(1)].[8]
ing substituents on the lower arene afforded signifi-
Based on these observations, we hypothesized that cantly higher conversions (95–99%) and a/b product
a strategic change in the nature of the phosphine ratios ranged from 30:70 for XPhos (entry 3), 10:90
ligand may facilitate this reductive elimination pro- for SPhos (entry 4) and 5:95 for DavePhos (entry 6).
cess and enable the general application of H-phos- It should be noted that the catalyst loading may be
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Palladium-Catalyzed Hydrophosphonylation of Alkenes
Table 1. Optimization of the reaction conditions for the pal-
ladium-catalyzed hydrophosphonylation of styrene with di-
butyl H-phosphonate.[a]
ing the reduction of Pd
um(0) species (entries 7 and 8). Indeed, a combination
of 6 mol% Pd(OAc)2, 6 mol% DavePhos and an addi-
ACHTUNGRTEN(NUNG OAc)2 to the active palladi-
AHCTUNGTRENNUNG
tional 6 mol% of 1c provided the desired product in
75% yield (a/b ratio of 6:94). Notably, this modified
protocol avoids the consumption of one equivalent
(with respect to Pd) of the dialkybiarylphosphine
ligand and overcomes one of the main drawbacks of
this catalyst activation protocol.
These optimized conditions were next applied to
the hydrophosphonylation of styrene with various
H-phosphonate derivatives (Table 2). Dimethyl and
diethyl H-phosphonate (1b and 1d, respectively) re-
acted similarly to dibutyl H-phosphonate 1c to afford
the hydrophosphorylated products 5b and 5d in good
yields and with excellent selectivity for the b-adducts.
Under these conditions, pinacol-derived H-phospho-
nate 1a reacted in high yield but a complete change
in selectivity for the a-adduct was observed. Indeed,
employing a catalyst system based on Pd/DavePhos,
product 5a was obtained in 80% yield with an 89:11
ratio of a/b isomers. Changing the ligand to SPhos
further improved the a/b-selectivity to 96:4. This
result further demonstrates the impressive reactivity
of derivative 1a. As well, these observations highlight
the ability of a single catalyst system to selectively
provide both a- and b-adducts based on the nature of
the H-phosphonate starting material. Finally, addition
of diphenyl H-phosphonate 1e to styrene was unsuc-
cessful under our standard reaction conditions.
[a]
Reaction conditions: PdACHTNUGRTENUNG(OAc)2 (6 mol%), ligand
(15 mol%), H2O (24 mol%), styrene (4a, 2.0 equiv.) and
dibutyl H-phosphonate (1c, 1.0 equiv.) in 1,4-dioxane
(0.5M) for 12 h at 1008C.
Table 2. Hydrophosphonylation of styrene with various
H-phosphonates.[a,b]
[b]
[c]
[d]
[e]
[f]
Determined by 31P NMR of the crude reaction mixture
using dimethyl H-phosphonate as an internal standard.
Isolated yields of product isomers after column chroma-
tography.
PdACHTUNGTRENNUNG(OAc)2 (3 mol%), SPhos (7.5 mol%) at 1008C for
36 h.
Addition of dibutyl H-phosphonate (1c, 1.0 equiv.) to the
catalyst mixture before the catalyst-activation protocol.
Addition of dibutyl H-phosphonate (1c, 1.0 equiv.+
6 mol%) to the catalyst mixture [PdACTHNUTRGENUG(N OAc)2 (6 mol%)
and DavePhos (6 mol%)] before the catalyst-activation
protocol.
decreased to 3 mol% palladium, although significantly
longer reaction times are required for complete con-
version of the H-phosphonate starting material
(entry 5). Based on the challenge in separating both
product isomers, DavePhos was chosen to pursue our
studies since it provided optimal regioisomer ratios.
Inspired by previous literature reports,[7d] we exam-
ined whether H-phosphonate 1c could be involved in
the water-promoted catalyst activation step, facilitat-
[a]
Reaction conditions: PdACHTNUTRGNENG(U OAc)2 (6 mol%), DavePhos
(6 mol%), H2O (24 mol%), styrene (4a, 2.0 mmol) and
H-phosphonate (1a–e, 1.0 mmol+6 mol%) in 1,4-diox-
ane (2.0 mL) for 12 h at 808C.
Isolated yields of product isomers after column chroma-
tography (a/b ratios were determined by 31P NMR of the
crude reaction).
[b]
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Mathieu Candy et al.
Table 3. Hydrophosphonylation of various alkenes with di-
and 5o were isolated in 37% and 53% yield, respec-
tively, after 48 h of heating at 1008C. Enol ether 5q,
1-methystyrene 5r, cyclopentene and cyclohexene
were unreactive.
butyl H-phosphonate.[a,b]
To overcome these limitations in scope with respect
to internal or disubstituted alkenes, we examined the
efficiency of our Pd-based catalyst system using the
more reactive pinacol-derived H-phosphonate 1a. For
these substrates, SPhos was found to be the optimal
ligand as previously highlighted with product 5a
(Table 2). It should also be noted that these hydro-
phosphonylation reactions generally lead to varying
amounts (15–25%) of pinacolphosphoric acid, result-
ing from the oxidation of 1a. Cyclic alkenes such as
cyclohexene and cyclopentene were found to react ef-
ficiently, affording the corresponding phosphonates 5s
and 5t in 70% and 64% yields, respectively (Table 4).
Cyclododecene proved less reactive. Incomplete con-
version of 1a was observed after 12 h and product 5w
was isolated in a modest 30% yield. Allylsilane was
well tolerated under these conditions, providing the
linear phosphonate 5x in 62% yield (a/b ratio of
9:91). Similar to our previous observations with re-
spect to the formation of 5a from styrene, high a-re-
gioselectivity was observed for the hydrophosphonyla-
tion of indene with pinacol-derived H-phosphonate
1a. Product 5y was obtained in 67% yield with an a/
b-selectivity of 94:6. Interestingly, E-b-methylstyrene,
which failed to react with 1c (5r, Table 3), provided
Table 4. Hydrophosphonylation of internal or disubstituted
alkenes with pinacol-derived H-phosphonate.[a,b]
[a]
Reaction conditions: PdACHTNUTRGNENG(U OAc)2 (6 mol%), DavePhos
(6 mol%), H2O (24 mol%), alkene (2.0 mmol) and dibu-
tyl H-phosphonate (1c, 1.0 mmol+6 mol%) in 1,4-diox-
ane (2.0 mL) for 12 h at 808C.
Isolated yields of product isomers after column chroma-
tography (a/b ratios were determined by 31P NMR of the
crude reaction).
[b]
[c]
The reaction mixture was heated at 1008C for 48 h.
Examples of the reaction scope for the hydrophos-
phonylation of various alkenes with dibutyl H-phos-
phonate 1c are found in Table 3. The reaction pro-
ceeded efficiently with several styrene derivatives, af-
fording the corresponding phosphonates in moderate
to good yields and with excellent anti-Markovnikov
selectivity. Electron-withdrawing and electron-donat-
ing substituents on the aromatic ring are well tolerat-
ed, and products 5f, 5g, 5k and 5l were obtained in
51–67% yield. Substitution at the ortho position, with
respect to the alkene, induced complete selectivity for
the b-adduct (5j and 5l, a/b>1:99), indicating that
steric hindrance can play a role in the reaction out-
come. Compared to styrene derivatives, aliphatic ter-
minal alkenes are less reactive. Linear products 5n
[a]
Reaction conditions: PdACHTNUGTRENUNG(OAc)2 (6 mol%), SPhos
(15 mol%), H2O (24 mol%), alkene (2.0 mmol) and pina-
col-derived H-phosphonate (1a, 1.0 mmol) in 1,4-dioxane
(2.0 mL) for 12 h at 808C.
Isolated yields of product isomers after column chroma-
tography (a/b ratios were determined by 31P NMR of the
crude reaction).
[b]
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References
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phonylation of alkenes with acyclic dialkyl H-phos-
phonate derivatives has been described. Dialkylbiaryl-
phosphine ligands have been employed to facilitate
the rate-determining reductive elimination step in
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Experimental Section
General Procedure for the Hydrophosphonylation of
Alkenes
A
Pd
dry Schlenk tube, under argon, was charged with
(OAc)2 (13.5 mg, 6 mol%) and DavePhos (23.6 mg,
U
6 mol%). It was then evacuated and backfilled with argon (3
times). Dry dioxane (2 mL) was added and the solvent was
degassed by three freeze, pump, thaw cycles. H-phosphonate
(1.06 mmol, 1.06 equiv.), and degassed (by sonication under
vacuum) water (4.3 mL, 24 mol%) were added and the reac-
tion mixture was heated at 808C for 90 seconds before addi-
tion of the alkene (2.0 mmol, 2.0 equiv.). The resulting solu-
tion was then heated at 808C for 12 h. After cooling, the
mixture was filtered over a pad of Celite, washed with dieth-
yl ether and the filtrate was concentrated. The crude materi-
al was purified by flash column chromatography (see the
Supporting Information for details).
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Acknowledgements
We thank the ANR (projet blanc 11-BS07-030), the Ambas-
sade de France en Pologne (MS, PhD grant), the CNRS, and
the ENSCM for financial support. We also thank Dr. C. Mid-
rier for insightful discussions.
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L. Montchamp, Adv. Synth. Catal. 2011, 353, 1883–
1888; d) S. Ortial, H. C. Fisher, J.-L. Montchamp, J.
Org. Chem. 2013, 78, 6599–6608.
c) A. N. Reznikov, N. K. Skvortsov, Russ J. Gen. Chem.
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[12] The synthesis of alkenes via the Horner–Wadsworth–
Emmons-like addition of deprotonated pinacol phos-
phonates to carbonyls is not possible. Generation of
the a-carbanion by deprotonation with strong bases
does not occur; instead, ring cleavage of the cyclic
phosphonate is observed. See: J. F. Reichweing, B. L.
Pagenkopf, J. Org. Chem. 2003, 68, 1459–1463.
[13] The reaction conditions for the hydrophosphonylation
of alkenes were optimized to obtain maximal conver-
sion of the H-phosphonate reagents since we found
that the purification of the products from any remain-
ing H-phophonate starting material was often quite
challenging. For this reason, two equivalents of alkene
are used and precautions to avoid oxygen in the reac-
tion mixture are necessary at this time.
[8] L.-B. Han, F. Mirzaei, C.-Q. Zhao, M. Tanaka, J. Am.
Chem. Soc. 2000, 122, 5407–5408.
[9] For the hydrophosphonylation of styrenes, allenes,
dienes and cyclopropenes, see: a) C.-Q. Zhao, L.-B.
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348, 1093–1100; e) B. K. Alnasleh, W. M. Sherrill, M.
Rubin, Org. Lett. 2008, 10, 3231–3234.
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[11] To the best of our knowledge, the hydrophosphonyla-
tion of isoprene with diethyl H-phosphonate has been
reported in 10% yield: a) T; Hirao, T. Masunaga, N.
Yamada, Y. Ohshiro, T. Agawa, Bull. Chem. Soc. Jpn.
1982, 55, 909–913. Three examples of the hydrophos-
phonylation of 2-allylmalonate with dialkyl H-phospho-
[14] For selected reviews, see: a) R. Martin, S. L. Buchwald,
Acc. Chem. Res. 2008, 41, 1461–1473; b) D. S. Surry,
S. L. Buchwald, Chem. Sci. 2011, 2, 27–50.
[15] B. P. Fors, P. Krattiger, E. Strieter, S. L. Buchwald, Org.
Lett. 2008, 10, 3505–3508. Four equivalents of water
(relative to palladium) are optimal for the reduction of
Pd(II) to Pd(0), as demonstrated by Buchwald and co-
workers. The use of two equivalents of water (relative
to palladium) led to incomplete catalyst activation and
subsequently incomplete conversion of the H-phospho-
nate starting material. A slight excess of water (eight
equivalents relative to palladium) was not detrimental
to the reaction.
nates using PdACHTNUGTRNEG(UN PPh3)4 have also been reported in 20–
26% yield: b) A. N. Reznikov, M. N. Sokolova, N. K.
Skvortsov, Russ. J. Gen. Chem. 2004, 74, 1461–1462;
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ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2014, 356, 2703 – 2708